I bought a 14 litres tank for my betta today and I was wondering how often should I change the water and how much amount. Have in mind that I've shut down the filter as it makes a lot of movement in the water.

If there's no filtration at all you'll probably have to replace all the water pretty frequently- as often as you need to to keep it from getting yucky.
On the other hand there are things you can do to lessen the current your filter produces if you think it's too strong for your fish.

Hm, I have 3 gallons, which is a little bit smaller than your tank, and I do about 20% water changes twice a week and 100% changes everyother week. I also have the filter on though. This seems to work well for me.
If the filter is off, you'll have to do it more than me.
Maybe you can use a plastic bottle to slow down the current, I'm sure someone else can provide the link that tells you how to do this method. It works quite well from what I understand.